A spark plug is mounted to a combustion apparatus, for example, an internal combustion engine, and is used for igniting an air-fuel mixture in a combustion chamber. Generally, the spark plug includes an insulator having an axial bore. A center electrode is inserted into a forward end portion of the axial bore. A metallic shell is provided externally of the outer circumference of the insulator. A ground electrode has a proximal end portion joined to a forward end portion of the metallic shell and a forward end portion forming a spark discharge gap in cooperation with the center electrode. Also, the metallic shell has an externally threaded portion for mounting the spark plug to the combustion apparatus. A solid annular gasket may be attached to the screw neck of the externally threaded portion (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2008-135370). When the spark plug is mounted to the combustion apparatus, the gasket provides airtightness between the spark plug (metallic shell) and the combustion apparatus.
Meanwhile, for example, if the spark plug is mounted to the combustion apparatus in such a positional relation that the ground electrode is present between a fuel injection device and the spark discharge gap, injected fuel hits against the back surface of the ground electrode. Accordingly, the presence of the ground electrode hinders the supply of an air-fuel mixture, potentially resulting in deterioration in ignition performance. Thus, according to a conceivable practice, the relative position of formation of a thread of the externally threaded portion in relation to a region of a forward end portion of the metallic shell where the ground electrode is fixed is set to a position corresponding to, for example, a thread-cutting start position of an internally threaded portion of a mounting hole of the combustion apparatus, whereby, when the spark plug is mounted to the combustion apparatus, the ground electrode is disposed at a fixed position in relation to a combustion chamber.
However, even when the thread of the externally threaded portion is formed at a predetermined relative position in relation to a forward end portion of the metallic shell (ground electrode), and the spark plug is mounted to the combustion apparatus with a predetermined tightening torque, the forward end portion of the metallic shell (ground electrode) may fail to be disposed at a fixed position in relation to the combustion chamber by the influence of, for example, frictional force generated between the gasket and the combustion apparatus.
The present invention has been conceived in view of the above circumstances, and an object of the invention is to provide a spark plug having a gasket which allows accurate establishment of the relative position of a forward end portion of the metallic shell (ground electrode) in relation to a combustion chamber in mounting the spark plug to a combustion apparatus, as well as an assembling structure of the spark plug in which the spark plug is mounted to the combustion apparatus.